Gas separation devices

ABSTRACT

A gas separation device for the removal of carbon oxides from a hydrogen-rich gas stream in the form of a composite comprising a hydrogen diffusion membrane and a methanation catalyst. The hydrogen diffusion membrane may be applied to the upstream surface of a porous or microporous support and the methanation catalyst may be applied to the downstream surface of the porous or microporous support. The gas separation device is particularly useful for negating pinhole leakages of carbon monoxide through palladium alloy membrane by virtue of the polishing-up action of the methanation catalyst

This application is the U.S. national-phase application of PCTInternational Application No. PCT/GB97/01477.

This invention relates to improvements in gas separation devices,particularly hydrogen diffusion membranes.

Hydrogen is one of the most important industrial gases. It is used, forexample, in ammonia synthesis, methanol synthesis, fuel cells, chemicalhydrogenations, gas chromatography, semiconductor processing, metalmanufacture, glass processing and also as a cooling medium in powerstations. In most of these applications, the hydrogen has to bevirtually 100% pure.

In recent years, synthetic permeable membranes have been developed whichcan be used for hydrogen separation and purification. A purificationtechnique which is based on the selective diffusion of hydrogen throughbundles of fine silver/palladium alloy tubes has been employed for someyears. However, this technique has not been universally accepted as agas clean-up device due to its extremely high cost, high operatingpressure and relatively low throughput. Improvements have been made inrecent years where a silver/palladium alloy is deposited on a ceramicsubstrate. This allows layers of metal to be made much thinner, therebyreducing cost and giving a higher specific throughput at a givenoperating pressure. Other membranes which have been developed forhydrogen separation and purification include ceramic membranes, zeolitemembranes and polymer membranes.

Palladium-based membranes are useful for a number of industrial andanalytical applications. One such application is the processing ofreformate gas streams to produce pure hydrogen for use in fuel cells.This hydrogen purification process has the advantage of being a singlestage process which is compatible with reformate gas streams andoperates at temperatures and pressures coincident with the normalreforming/cracking conditions (ie methanol, methane and otherhydrocarbons). The palladium alloy may be deposited on the poroussupport at desirable thicknesses using a variety of methods, of whichsputtering, chemical vapour deposition, physical vapour deposition andelectroless plating are examples.

Although supported palladium alloy membranes of the above type, withvery high specific flow rates, have been manufactured with some degreeof success, the coating of a porous ceramic support with an essentiallynon-porous thin alloy film represents special problems. Small defects inthe support lead to pin-holes in the palladium alloy membrane whichcompromise the maximum hydrogen purity which such composites can attain.Furthermore, the very important application of hydrogen processing forfuel cells requires hydrogen with a very low carbon monoxide content(typically less than 100 ppm for low-temperature phosphoric acid fuelcells and less than 10 ppm for proton exchange membrane fuel cells).This gas quality is close to the current state of the art for supportedpalladium alloy membranes and for such a critical application anadditional degree of security is required. There is currently a highfailure rate in the production of totally leak-free supported palladiumalloy membranes and if a leak develops during use of such a membrane ina fuel cell system, the increased level of carbon monoxide can have animmediate poisoning effect on the fuel cell anode catalyst.

European Patent No 0434562 B1 relates to a process and apparatus for thepurification of hydrogen gas streams used for hydrogenations in refineryand petrochemical plants. The carbon monoxide in such purified hydrogengas streams needs to be less than 50 ppm. In this purification process,the hydrogen stream to be purified is firstly applied to a gas diffusionmembrane capable of preferentially allowing hydrogen to pass through andat the same time preferentially blocking other components of the gasstream such as carbon monoxide and hydrogen sulphide. Most of the carbonmonoxide in the original hydrogen stream is removed by the membrane buta small amount of carbon monoxide passes through the membrane. Thepermeate gas stream is then subjected to a subsequent and separate stageof methanation wherein the carbon monoxide content is lowered to therequired level. The process and apparatus described in theaforementioned European Patent is intended for large scale industrialoperation at high pressures and high flow rates. Feed gas pressures of40 to 120 bars, pressure drops of 30 to 80 bars and flow rates of 12,700Nm³/hr (ie over 200,000 liters/min) are mentioned. Moreover, because ofthe large volume of hydrogen involved, the two-step purification process(gas diffusion and methanation) is suitably conducted in several stages.Also, the gas diffusion and methanation steps are conducted at differenttemperatures.

The present invention provides an improved process and apparatus for thepurification of hydrogen gas streams by a combination of gas diffusionmembrane and methanation.

The present invention also provides a gas separation device whichovercomes the problems of current gas diffusion membranes by preventingleakage of carbon oxides through the membrane.

According to the present invention there is provided a gas separationdevice in the form of a composite comprising a hydrogen diffusionmembrane and a methanation catalyst for the removal of carbon oxidesfrom hydrogen gas streams.

Suitably, the hydrogen diffusion membrane is associated with theupstream surface of a porous or microporous support and the methanationcatalyst is associated with the downstream surface of the porous ormicroporous support.

Preferred support materials include alumina and alumino silicates.

Suitable hydrogen diffusion membranes include palladium alloy membranes,ceramic membranes, zeolite membranes and polymer membranes. Examples ofceramic membranes are a porous glass membrane marketed under the trademark “Vycor” which does not require a ceramic support and a metal oxidemembrane marketed under the trade mark “Velterop”. Examples of polymermembranes are polyimides and polysulfone membranes marketed under thetrade mark “Prism”.

Preferred palladium alloy membranes are palladium alloyed with one ormore metals selected from Ag, Au, Pt, Cu, B, In, Pb, Sn and rare earths.

The palladium alloy membrane is preferably from 1 to 10 microns thick.

In the gas separation device of the present invention the methanationcatalyst preferably is a selective methanation catalyst for the removalof carbon monoxide and/or carbon dioxide from a hydrogen gas stream.

Suitable methanation catalysts are those based on iron, cobalt, nickel,ruthenium, rhodium, palladium, osmium, iridium or platinum.

In the case of carbon monoxide removal from reformate gas mixtures,particularly for fuel cell applications, the methanation catalystsuitably should be capable of reducing carbon monoxide in the permeategas stream to a concentration below 100 ppm, preferably below 10 ppm.

A further aspect of the invention provides a process for thepurification of a hydrogen gas stream using the gas separation device asclaimed herein.

Suitably, the hydrogen gas stream to be purified is a reformate gasmixture.

Preferably, the hydrogen gas stream is fed to the gas separation deviceat a pressure less than 30 atmospheres.

Further preferably, the pressure drop of the hydrogen gas stream overthe gas separation device is less than 15 atmospheres.

Suitably, the flow rate of the hydrogen gas stream fed to the gasseparation device is less than 10,000 liter/min.

Suitably, the hydrogen diffusion membrane and the methanation catalystfunction within the same temperature window.

Suitably, also the hydrogen gas stream is purified in a single passthrough the gas separation device.

The present invention is also a fuel cell system for vehicularapplication comprising (a) an on-board hydrogen supply unit; (b) ahydrogen purification unit and (c) a fuel cell wherein the hydrogenpurification unit comprises a gas separation device as claimed hereinand operates by the process claimed herein.

Suitably, the fuel cell is a proton exchange membrane fuel cell or alow-temperature phosphoric acid fuel cell.

Embodiments of the invention will now be described by way of exampleonly.

EXAMPLE 1

Sample Construction

A hydrogen diffusion membrane was prepared by applying a 7.5 micronPd-Ag film on the outside of a porous alumino-silicate ceramic tube (7cm long and 1.55 cm diameter) by the technique of electroless plating. A2.6% Rh/alumina methanation catalyst supported on a ceramic monolith(cordierite) was inserted into the ceramic tube.

The catalyst was prepared by applying rhodium nitrate solution(containing 67 mg Rh) to an alumina washcoat located on a 39 mm lengthof monolith segment, which had been shaped to fit the ceramic tube. Themonolith segment was calcined in air at 500° C., reduced with aqueousNaBH₄ solution, and dried, before being cemented inside the ceramictube.

Measurements

The membrane/methanation catalyst composite was sealed into a moduleusing graphite rings giving an effective area (of exposed membrane) of24 cm². A synthetic reformate (containing 70% H₂, 28% CO₂ and 2% CO) wasapplied at pressure (3 atm) and elevated temperature (440° C.) to theoutside of the membrane, and the resulting flow of gas through themembrane/methanation catalyst composite was measured. The composition ofthe permeated gas stream was analysed by gas chromatography.

The module was then subjected to a thermal cycle that is known todegrade the membrane and reduce its resistance to permeation by CO andCO₂. The thermal cycle consisted of cooling the module to roomtemperature and re-heating to 440° C., with 0.5 bar differentialpressure of neat hydrogen across the membrane. The synthetic reformatewas then applied again to the membrane/methanation catalyst composite(as above).

Results

The rate of gas permeation through the fresh membrane/methanationcatalyst composite was 600 cm³ min⁻¹. Apart from hydrogen, the exit gascontained 156 ppm CH₄, 2 ppm CO and 81 ppm CO₂ As CH₄ was absent fromthe synthetic reformate, its presence in the exit stream indicated thatthe methanation catalyst was active in the removal of carbon oxides thatwere permeating through the membrane.

After the thermal cycle, the gas permeation rate was still the same, butthe CH4 concentration in the exit stream was slightly higher (162 ppm);the CO and CO₂ concentrations were 2 and 22 ppm respectively.

EXAMPLE 2

Sample Construction

The membrane/methanation catalyst composite was prepared as in Example1, except that the rhodium methanation catalyst was supported directlyon the inside of the alumino-silicate ceramic tube supporting themembrane. The catalyst was applied by treating the inside of the ceramictube with 0.5 cm³ of rhodium nitrate solution, before drying andreducing as in Example 1. As a result, the ceramic tube contained 56 mgof rhodium.

Measurements

The membrane/methanation catalyst component was fitted into a module andtested as in Example 1. After the initial test, however, themembrane/methanation catalyst composite was subjected to 5 thermalcycles, before being re-tested.

Results

The initial gas permeation rate was 600 cm³ min⁻¹. The exit stream waspredominantly hydrogen, except for 139 ppm CH₄ and 8 ppm CO₂; no CO wasdetected.

After the 5 thermal cycles, the exit stream contained 430 ppm CH₄,indicating that substantial degradation of the membrane had occurred.However, the amount of carbon oxide that was able to pass through therhodium catalyst without being methanated was very low. Only 0.6 ppm COand 4 ppm CO₂ were detected.

Notable features of the present invention include those listed below.

1. The process and apparatus of the invention are particularly suitablefor small scale operations (eg less than 600 liters/min of hydrogen-richfeed gas).

Examples of such small scale applications include (a) a system forsupplying hydrogen or a hydrogen-rich gas to fuel cells and (b) supplyof pure hydrogen for gas chromatography and other instruments.

If hydrogen was to be used say for an analytical instrument then therequirement would be for high purity and the flow rate would be very low(say one (1) liter/min or less). Furthermore, a typical flow rate forthe hydrogen feed gas in a vehicular fuel cell application would be 180m³/hour.

2. The process and apparatus of the invention are also suitable foroperation at comparatively low pressures, eg less than 40 bar on thefeed side and a pressure drop typically of up to a few atmospheres. Forexample, in the analytical instrument example mentioned above, thepressure required to the instrument would probably be about 2 bar. Inthe case of fuel cells less than 5 bar pressure would be required.

3. The present invention is also suitable for both “static” and“portable” (mobile) applications. The former include phosphoric acidfuel cells and the latter includes low temperature low-temperaturephosphoric acid fuel cells and proton exchange membrane fuel cells.

4. In small scale applications, the present invention allows forpurification of the hydrogen feed stream in a single pass through thegas separation device although for large scale applications two or morepurification stages may be required.

5. The present invention is also suitable for removing carbon dioxidefrom hydrogen-rich gas streams as well as the removal of carbonmonoxide.

6. The apparatus and process of the present invention are also suitablefor operation with the same absolute pressure but different partialpressures on either side of the membrane/catalyst composite.

7. A major advantage of the present invention is that it provides a gasseparation device which negates pin-hole leakages of carbon oxidesthrough a hydrogen diffusion membrane by means of a combination of themembrane and methanation catalyst in the form of a composite. As aresult, a lower specification can be set for the manufacture of thehydrogen diffusion membrane and also slight deterioration of a perfectmembrane will not necessarily impact on the rest of the system. This isof particular importance with regard to palladium alloy membranes.

What is claimed is:
 1. A gas separation device comprising a hydrogendiffusion membrane which is associated with an upstream surface of aporous or microporous support and a finely divided methanation catalystfor the removal of carbon oxides from hydrogen gas streams, saidmethanation catalyst being associated with a downstream surface of thesupport, wherein the hydrogen diffusion membrane, the support, and themethanation catalyst are in the form of a composite.
 2. A gas separationdevice according to claim 1 wherein the support material is alumina oran alumino-silicate.
 3. A gas separation device according to claim 1wherein the hydrogen diffusion membrane is a palladium alloy membrane, aceramic membrane, a zeolite membrane or a polymer membrane.
 4. A gasseparation device according to claim 3, wherein the palladium alloymembrane comprises palladium alloyed with one or more metals selectedfrom the group consisting of Ag, Au, Pt, Cu, B, In, Pb, Sn and the rareearths.
 5. A gas separation device according to claim 4, wherein thethickness of the palladium alloy membrane is from 1 to 10 microns.
 6. Agas separation device according to claim 1 wherein the methanationcatalyst is a selective methanation catalyst for the removal of carbonmonoxide and/or carbon dioxide from a hydrogen gas stream.
 7. A gasseparation device according to claim 6 wherein the methanation catalystis based on iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium,iridium or platinum.
 8. A process for the purification of a hydrogen gasstream using a gas separation device as claimed in claim
 1. 9. A processaccording to claim 8 wherein the hydrogen gas stream to be purified is areformate gas mixture.
 10. A process according to claim 8 wherein thehydrogen gas stream is fed to the gas separation device at a pressureless than 30 atmospheres.
 11. A process according to claim 8 wherein thepressure drop of the hydrogen gas stream over the gas separation deviceis less than 15 atmospheres.
 12. A process according to claim 8 whereinthe flow rate of the hydrogen gas stream fed to the gas separationdevice is less than 10,000 liters/min.
 13. A process according to claim8 wherein the hydrogen gas stream is purified in a single pass throughthe gas separation device.
 14. A process according to claim 8 whereinthe hydrogen diffusion membrane and the methanation catalyst of the gasseparation device function within similar temperature ranges.
 15. A fuelcell system for vehicular application comprising (a) an on-boardhydrogen supply unit; (b) a hydrogen purification unit and (c) a fuelcell wherein the hydrogen purification unit operates by the process inaccordance with claim
 8. 16. A fuel cell system for vehicularapplication comprising (a) an on-board hydrogen supply unit; (b) ahydrogen purification unit and (c) a fuel cell wherein the hydrogenpurification unit comprises a gas separation device as claimed inclaim
 1. 17. A fuel cell system according to claim 16 wherein the fuelcell is a proton exchange membrane fuel cell or a low-temperaturephosphoric acid fuel cell.